Problems using an internal modem with MSComm control 
Author Message
 Problems using an internal modem with MSComm control

Hi,

I'm trying to debug a problem with the MSComm control. An application I'm
looking at talks to an embedded device via the serial port and a modem. The
program works fine when used with an external modem, but drops characters
when using an internal modem. I have tried this on different machines with
different modems, so I don't think this is related to the hardware. I have
played with the handshaking control (XON/XOFF and RTS) but with no luck. The
characters that are dropped from the string are random. I've also tried the
MSComm control sample program that comes with VB and it also does the same
thing, drops characters from this device when used with an internal modem.

Does anyone have any ideas about what might be going on? Or, any differences
you might know when using an internal modem verus an external one?

Thanks in advance,
Doug Farrell



Thu, 24 May 2001 03:00:00 GMT  
 Problems using an internal modem with MSComm control
Hi Doug,

I design, prototype, program, and debug Embedded Controllers.  I've had
about every posible bug show up over the years in writing the Serial Comm
code in VB to talk to various embedded devices.

The first qustion that comes to mind is; Do you have 2 Comm controls in
the one project?  If so, is one dedicated to the external modem and the
other to the internal modem?  I take it that you are using only one
modem at a time.  Using more than one at a time is not a problem.  The
problem may be that the code for on_comm, if you are using this, may
differ from one control to the other.

Another may be a difference in the settings of the "S" Registers of the
2 modems.  Read through the documentation of each modem.  One problem
that really caused a loss of time here a few years back was the
initialization of an internal modem to function as a Fax/Modem.  The
problem disappeared upon Intialization of this modem to function as
a Modem, rather than a fax modem.

You may find that the raw data being passed to the modem has certain
characters which a fax modem sees as escape codes.  I may be wrong
on this.

Are you running the code as "Event Driven" such as using "On_Comm"
or are you "Polling" with a timer?  There are many ways to get the
data from the modem.  I have a few hints, and can more easily convey
my thoughts/experiences via a phone conversation.  I will send a
separate email to you with my business phone number.

Try out the above(checking for fax/modem init and the s-regs differences)
If that does not take care of the problem call or email me.  I will do
all that I can to help.  Been in your position, and really know what a
pain this stuff can be.

Good luck with it.

Brent

-------

Quote:

> Hi,

> I'm trying to debug a problem with the MSComm control. An application I'm
> looking at talks to an embedded device via the serial port and a modem. The
> program works fine when used with an external modem, but drops characters
> when using an internal modem. I have tried this on different machines with
> different modems, so I don't think this is related to the hardware. I have
> played with the handshaking control (XON/XOFF and RTS) but with no luck. The
> characters that are dropped from the string are random. I've also tried the
> MSComm control sample program that comes with VB and it also does the same
> thing, drops characters from this device when used with an internal modem.

> Does anyone have any ideas about what might be going on? Or, any differences
> you might know when using an internal modem verus an external one?

> Thanks in advance,
> Doug Farrell



Sat, 26 May 2001 03:00:00 GMT  
 
 [ 2 post ] 

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